Hexakis (alkylisonitrile) technetium(I) complexes are a new class of cationic, lipophilic myocardial perfusion imaging agents. To further evaluate the effect of lipophilicity on myocardial uptake characteristics, the authors systematically synthesized and tested Tc-isonitrile complexes of varying lipophilicity in both cellular and whole animal systems. In chick heart cells in monolayer culture, cellular plateau level uptake in general correlated with lipophilicity of the complexes (determined by reverse phase high performance liquid chromatography) (r = .71) as well as with scintigraphic intensity of imaged rabbit hearts (r = .91). Exceptions to this trend indicated that additional factors such as size of the complex and form of the terminal alkyl chain branching also may have influenced uptake. The data indicated that neither the lipophilic properties nor the cation charge alone were sufficient to predict myocardial uptake. In addition, intravenous injection of complexes into rabbits showed optimal myocardial images with agents of intermediate lipophilicity. Results indicated that, following intravenous administration, complexes of low lipophilicity yielded suboptimal myocardial images because of low heart cell uptake, whereas complexes of high lipophilicity yielded poor relative myocardial visualization because of excessive binding to additional organs and compartments.